All I can say is every time you use a diopter or extension tube you'll be kicking yourself for not buying a macro lens.
Anything you stick on the lens is going to cut the light through the lens. When doing close in shooting, even at 2 ft, depth of field is everything and that means light, lots of it. So anything that slows down a lens is anathema to getting maximum light through the lens.
That is true. I understand that logic. I don't know how technology has advanced in terms of diopter technology though. Are the modern "expensive" ones capable of passing light through as well as plain UV filters. The reviews for the Canon 500D
on amazon are quite good.
And, speaking of kicking myself, I've learned that when entering a hobby to not buy the beginner set as I will outgrow it quickly. So, part of me wants to overkill on the lens, so I never want to upgrade again. I am just questioning how often will I use a macro lens because if I get one then it will be used only for the tank.